The invention relates to a suction device for cleaning purposes having a collecting container, which comprises a closable suction inlet, a suction outlet and a filter disposed therebetween and is connected to a suction unit by a suction line emanating from the suction outlet, the suction line being closable by means of an outlet valve and an external air supply closable by means of a supply valve being disposed at the side of the filter remote from the suction inlet.
Such devices are used, for example, as industrial dust extractors. After a certain operating period the filter becomes extremely dirty, so that it becomes necessary to dedust the filter. In simple devices, this is effected by knocking the dust off the filter, e.g. with the aid of a mechanical scraper.
EP-B1-0 197 036 proposes the use, in a dust extractor, of two separate filters as well as a device for reversing the air circulation in a respective one of the filters. For said purpose, the suction line is closed and the air drawn in through the one filter flows through the other filter in reverse flow direction so that a dedusting of said filter is effected. This requires a construction of an elaborate design as well as the provision of two separate filters.
In DE-C1-41 38 223 it is proposed to effect the filter dedusting in a plurality of stages, during each of which one sub-region of the filter is covered by an external air supply so that said filter sub-region is swept in reverse flow direction and hence cleaned. Cleaning of the entire filter is extremely time-consuming and, in addition, the external air supply directable onto the individual sub-regions requires a considerable constructional outlay.
It is also known (DE-OS 21 06 058) for the area between filter and suction unit to be connected by a valve opening to outside air and for the valve opening to be alternately opened and closed during simultaneous operation of the suction unit. The filter is clamped onto a rigid supporting frame and as a result of the pressure variations associated with opening and closing of the valve opening the filter is knocked against its supporting frame and dust particles adhering to the filter are knocked off. Knocking-off however effects only an inadequate dedusting of the filter as the filter is not sufficiently swept in reverse flow direction.
In DE-AS 12 45 550 it is proposed to achieve filter dedusting by first closing the suction inlet while the suction unit is in operation in order to produce a partial vacuum in the collecting container. Then the suction unit is switched off and/or the outlet valve closed. A supply valve is then opened and, by means of an external air supply, outside air is admitted into the area between filter and outlet valve. The mode of operation is therefore very complicated and susceptible to operating errors and, in addition, adequate filter dedusting is also not always achieved.
U.S. Pat. No. 4,277,265 describes a suction device which uses a bag-shaped filter, which is pressed together during dedusting of the filter. For said purpose, an inlet valve and an external air supply closable by means of a supply valve are provided in a collecting container which accommodates the filter, the inlet valve being mechanically coupled to the supply valve. When the inlet valve is closed, the supply valve is simultaneously opened and external air flows at the side of the collecting container remote from the filter into the collecting container. The bag-shaped filter is therefore pressed together and the air remaining in the filter upon closing of the inlet valve may escape through the suction outlet. With said suction device, it is necessary for the suction unit to be flow-connected to the collecting container during filter dedusting, i.e. when the supply valve is opened and external air flows in. Filter dedusting is therefore associated with a high loading of the suction unit.